An ink jet printer generally comprises at least one nozzle and an ink liquid supply system which supplies ink liquid to the nozzles at an appropriate pressure so that a jet of ink liquid is ejected onto a recording medium such as a sheet of paper. An ultrasonic transducer is usually provided for generating ultrasonic waves in the ink liquid to assist the separation of the ink jet into a sequence of equal-sized droplets. In a typical ink jet printing system, the droplets are electrically charged by passing the drops through a ring electrode and thereafter, through a pair of deflection plates. The charged droplets are selectively deflected in response to a voltage applied to the deflection plates, so that the droplets either impinge on the sheet of paper or are deflected to a beam gutter. The ink collected by the beam gutter may be recirculated to the supply system.
U.S. Pat. No. 4,135,197 discloses an ink jet printing device having a printing head which comprises a plurality of printing nozzles. The printing head includes an elongated chamber serving as an ink reservoir. One of the walls of the chamber is formed by an orifice plate which contains the nozzle orifices. An ink supply system is adapted to supply ink to the reservoir and to maintain a constant pressure in the reservoir so that the ink is distributed equally to the different orifices.
The ink jet printing system so disclosed has the advantages of a large number of ink jets that can be generated simultaneously so that the printing speed can be considerably increased. However, a reliability problem is encountered when the nozzles become clogged, for example, with dried ink. If the nozzles are partly clogged, deviations in the effective cross sections of the nozzles result in nonuniform flow rates of ink liquid through the different nozzles. This results in an impairment of the quality of the printed image.
In connection with single-nozzle ink jet printers. U.S. Pat. No. 4,263,602 discloses an ink liquid supply system which employs a constant flow rate pump. However, even if such a constant flow rate pump were employed in a multi-nozzle system such as that described in U.S. Pat. No. 4,135,197, the tendency of the nozzles to become clogged would not be eliminated and the reliability of the system would continue to be poor.
Accordingly, it is an object of the present invention to provide a multi-nozzle ink jet printer which has an improved reliability.